Add NavigationToFirstContentfulPaint UMA for ServiceWorker controlled pages.

components/page_load_metrics/renderer/metrics_render_frame_observer.cc

Index: components/page_load_metrics/renderer/metrics_render_frame_observer.cc
diff --git a/components/page_load_metrics/renderer/metrics_render_frame_observer.cc b/components/page_load_metrics/renderer/metrics_render_frame_observer.cc
index 260d3b6285d48c68a63ce22b127e9b70890555ad..311d857a2066b369c15323c41cb789cb835d96de 100644
--- a/components/page_load_metrics/renderer/metrics_render_frame_observer.cc
+++ b/components/page_load_metrics/renderer/metrics_render_frame_observer.cc
@@ -41,8 +41,8 @@ void MetricsRenderFrameObserver::DidChangePerformanceTiming() {
 
 void MetricsRenderFrameObserver::DidObserveLoadingBehavior(
     blink::WebLoadingBehaviorFlag behavior) {
-  DCHECK(page_timing_metrics_sender_);
-  page_timing_metrics_sender_->DidObserveLoadingBehavior(behavior);
+  if (page_timing_metrics_sender_)

[Charlie Harrison, 2016/04/13 12:59:13]

This worries me slightly. We try to scope page_timing_metrics_sender_ to a
committed page, so that we never send IPC for a provisional load.

If you are sometimes calling DidObserveLoadingBehavior before the commit point
(from contents point of view), then won't you miss this window occasionally and
not update the behavior?

Looking at the code now, it seems like you're instrumenting after content should
know we've commit. Still, can you add a comment explaining when the sender_ will
be invalid?

[horo, 2016/04/14 02:19:13]

DidCommitProvisionalLoad() doesn't create page_timing_metrics_sender_ if
ShouldSendMetrics() returns false.
For example, when the mime type isn't HTML mime type,
page_timing_metrics_sender_ is null when DidObserveLoadingBehavior() is called.

So I think we should check ShouldSendMetrics() or page_timing_metrics_sender_
here.
But as you say, checking only page_timing_metrics_sender_ may cause IPCs for a
provisional load.
So I changed to check ShouldSendMetrics() here.

+    page_timing_metrics_sender_->DidObserveLoadingBehavior(behavior);
 }
 
 void MetricsRenderFrameObserver::DidCommitProvisionalLoad(